I have a box of old Agfa scientific plates no idea what they were for, and can't remember where I got them, I just opened the box in the end in daylight and they deteriorated bdly anyway. Tey are the perfect width for the one 9x12 which is missing a screen, I think approx 9 x 28 cm, so I could get 2 screens out of each plate. I'll make spare glass inserts for my Durst enlargers at the same time. As a penniless student in the earky 70's I made some from FP4 plates. There still perfect
Not sure how different in size the focus screens are compared to 9x12 plates. I'll check next time I'm in the UK.